The use of optical fiber for the transmission of communication signals is rapidly growing in importance due to its high bandwidth, low attenuation, and other distinct advantages such as radiation immunity, small size, and light weight. In optical communication networks, it is often necessary or desirable to split the optical signal into parts having either predetermined proportions of the original signal intensity in order to monitor the state of the communication system, or to split optical frequencies for multiplexing independent signals. A device having the capability of extracting a portion of the optical signal intensity from a communication channel is herein referred to as an optical tap coupler, and a device capable of combining or separating optical frequency components in an optical communications signal is herein referred to as a wavelength division multiplexer (WDM).
Presently, and in future optical networks, the desired properties of optical tap couplers include tight control of the proportions of the split optical signals, low insertion loss, and minimizing variations in modal and polarization states as well as signal spectrum. In addition, in communication systems where higher data rates are achieved by transmitting aggregated data rates over parallel fibers, it is important to reduce the form factor of optical tap couplers in order to achieve high density in optical networks employing the monitoring function.
State of the art optical tap couplers do not provide the means of achieving all of the above mentioned desirable characteristics. The most commonly used technology based on fused biconical tapered optical fibers are spectral and modal dependent. Therefore, when these tap couplers are installed in optical networks utilizing multimode fiber, a significant degradation in the network performance results. Other optical taps technologies based on lensing and filtering technology have reduced or negligible degradation on network performance; however, they require multiple components to support channels operating over parallel transmission lanes. Deploying multiple optical tap couplers for channels utilizing parallel optics significantly increases system cost and size, while reducing channel reliability. In practice, size and cost scale with the number of parallel fibers. For example, the transmission of 100 Gbps Ethernet over 10 parallel lanes of multimode fiber (100GBASE-SR10), utilizing full duplex optical tap couplers based on filter technology, requires more than 40 lenses, 20 filters, and other multiple components.
In traditional fiber network systems, the optical transmitter on one end is connected to the optical receiver at the other end via a fiber optic cable. With such a system, two fibers are required to complete a full duplex circuit, one fiber connects an optical transmitter at the near end to an optical receiver at the distal end and the other fiber connects an optical receiver at the near end to an optical transmitter at the distal end. Bi-directional fiber optic transceivers, on the other hand, are capable sending and receiving optical signals on only one fiber. The signal moving in one directional is transmitted at a different wavelength from the signal moving in the opposite direction. Although only a single fiber is required, in a typical bi-directional system, a second fiber may be used to double the traffic capacity.